1. Field of the Invention
The present invention relates to an apparatus for emitting and receiving light, said apparatus comprising an emitter with a spectrally broadband light source and a concave first mirror in the focus of which said light source is positioned, and a receiver which is adapted to receive light emitted from the light source and which is connected to equipment for analyzing the light received.
2. Description of Background Art
The paper "Differential optical absorption spectroscopy system used for atmospheric mercury monitoring", in Applied Optics, Vol. 25, No. 3, Feb. 1, 1986, reports on equipment for controlling air pollution by means of differential optical absorption spectroscopy. This equipment is chiefly made up of a device for emitting light, a device for receiving the light emitted, a spectrometer, a multi-channel analyser, and a computer. The device for emitting light (in the following called the emitter) comprises a spectrally broadband light source and a concave mirror in the focus of which the light source is positioned. The device for receiving the light emitted (in the following called the receiver) comprises a concave mirror and an inclined plane mirror which is arranged in the vicinity of the focus of the concave mirror and which reflects the received light to the input optics of the spectrometer. Emitter and receiver should be spaced apart a distance of from 10 m to 10 km.
When measuring air pollution by means of this equipment, a parallel light beam is emitted from the emitter to the receiver through the area in which the air pollution is to be measured. The light received is spectrally divided in the spectrometer and the resulting spectrum is fed to the multi-channel analyser and the computer by means of which the concentration C of different substances in the air between the emitter and the receiver can be determined on the basis of the Lambert-Beer law: EQU C=log (I'.sub.o /I)/(.epsilon.L).sup.2
wherein
C is the concentration of a substance, PA1 I'.sub.o is the light intensity without any differential absorption, PA1 I is the light intensity depending on the absorption of the substance, PA1 .epsilon. is the differential absorption cross-section of the substance, which is determined by calibration, and PA1 L is the measuring distance.
The quotient (I'o/I) is determined by dividing the spectrum obtained from the spectrometer by a polynomial (first to fifth order) which is obtained by a least squares fit to the absorption spectrum.
However, the apparatus described is in some respects inadequate. First, it can only by used for differential measurings, which means that only the concentration of substances which absorb light at one or a few distinct wavelengths or wavebands can be determined. Thus, the concentrations of Cl.sub.2, H.sub.2 S and other substances which absorb light continuously or in broad wave length ranges cannot be determined by means of this equipment. In order to determine the concentration of these substances, total absorption measurings have to be effected.
Second, the receiver is difficult to mount because it has to be fixed in a highly exact position so that the light can be reflected to the plane mirror and further to the input optics of the spectrometer. In some weather conditions, the receiver may be dislocated and require adjustment.
Third, there has to be two power supplies, one for the emitter and one for the receiver.
Fourth, it is difficult in narrow measuring spaces, e.g. in chimneys, to place the emitter and the receiver at a sufficient distance from one another to make the measuring distance long enough.